Return number of gridcells, landunits, columns, and pfts across all
processes.

USES:

ARGUMENTS:

implicit none
integer, intent(out) :: ng ! total number of gridcells
! across all processors
integer, intent(out) :: nl ! total number of landunits
! across all processors
integer, intent(out) :: nc ! total number of columns
! across all processors
integer, intent(out) :: np ! total number of pfts
! across all processors